This invention relates to a front and rear wheel steering device for vehicles such as four-wheel vehicles.
The present inventors have previously proposed a steering device for vehicle which steers the rear wheels in relation with the steering of the front wheels according to vehicle speed in copending U.S. Pat. applications Nos. 821,998, 822,000, 822,008, 822,010, 822,043, and 822,293, all of which were filed on Jan. 24, 1986 and assigned to the same assignee. According to these devices, rear wheels are generally steered in the same phase relationship or none at all in high speed range and are steered in the opposite phase relationship in low speed range. The steering angle ratio is a continuous function of the vehicle speed, with the steering angle ratio assuming a positive value or a same phase relationship when the vehicle speed is greater than a certain value and assuming a negative value or a opposite phase relationship when the vehicle speed is lower than it (refer to "Q" in FIG. 5), whereby the functionality of the steering device is adapted to both high and low speed ranges.
As a result, the minimum angle of turning and the inner radius difference of the vehicle are both drastically reduced and the maneuverability of the vehicle, particularly in low speed range, in driving the vehicle into a garage, driving the car through narrow and crooked alleys and making a U-turn, is substantially improved with the additional advantage of improving the dynamic lateral response of the vehicle in high speed range, for instance in changing driving lanes.
Such a four wheel steering system is highly convenient in driving through narrow streets and parking the car in a limited space in low speed range by setting the steering ratio to the opposite phase relationship and reducing the inner radius difference, but it has the disadvantage that the rear part of the vehicle is swung laterally when turning and parking the car along a curbstone, and changing the lateral position of the vehicle in a limited space become difficult. Therefore, it is desired that the steering angle ratio may be manually set to about 1.0 in the same phase relationship in low speed range when necessary.
In such a case, however, the driver must release the manual selection before the vehicle speed has increased beyond a certain level since the maneuvering of the vehicle, in particular turning the vehicle, becomes difficult if the steering ratio is about 1.0 in the same phase relationship, and such a necessity to release the manual setting is burdensome to the driver because failure to release the manual setting will cause difficulty in maneuvering the vehicle particularly in low to medium speed range.